The topic of “Further Enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) Interference Management and Traffic Adaptation” has been agreed as a study item in 3GPP release 11 and a work item in 3GPP release 12. Performance evaluation of various deployment scenarios has been conducted by both 3GPP RAN 1 and RAN 4 working groups. It has been shown that an average cell throughput can be improved to a large extend by allowing dynamic reconfigurations in Long Term Evolution (LTE) time division duplex (TDD) systems.
The TDD scheme would offer flexible deployments without requiring a multiple spectrum resources. Currently, the LTE TDD would allow for asymmetric downlink-uplink (DL-UL) allocations by providing seven different semi-statically configured DL-UL configurations as illustrated in FIG. 1, and these allocations can provide between 40% and 90% DL subframes. To be more specific, the seven different semi-statically configured DL-UL configurations are indexed in the left most column of FIG. 1 and are numbered between 0˜6. In the present disclosure, a DL-UL configuration is also referred to as a TDD configuration or a TDD DL-UL configuration. For each TDD configuration, subframes of a radio frame would be configured as a downlink subframe, as an uplink subframe, or as a special subframe, and the top row of FIG. 1 shows the index of the subframe numbers. Therefore, in order to configure a radio frame to have a certain number of downlink and uplink slots, an evolved Node B (eNB) would transmit one of the UL-DL configurations in system information (SI).
For example, if heavy downlink traffic has been experienced by the network, the eNB could decide upon the TDD configuration 5 which would be transmitted to UEs and would provide 8 downlink slots and 1 uplink slot per radio frame. However, if heavy downlink traffic has all in a sudden been changed to heavy uplink traffic, the eNB may not change the TDD configuration instantly but has to convey the change by modifying the system information, and the modification of the system information for a legacy UE could only occur at a modification boundary. This would mean that the re-configuration of the TDD configuration via the SI change would be semi-static rather than dynamic and may not match the instantaneous traffic situation.
In comparison to the system information change procedure, known dynamic re-configuration techniques would require a much shorter period for TDD reconfiguration. Evaluation in the corresponding study item reveal significant performance benefits by allowing TDD DL-UL reconfiguration based on traffic adaptation in small cells as mentioned in “Further enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) interference management and traffic adaptation,” 3GPP TR 38.828, V11.0.0, 2012-06, which is incorporated by reference for definition purposes. Also, it was shown that a dynamic signaling mechanism would outperform the mechanism that uses the system information change procedure.
Also for definition purposes, the TDD frame structure, DL-UL configurations, and the UL-HARQ timing relations could be defined according to “Physical Channels and Modulation,” 3GPP TS 36.211, V11.0.0, 2012-09 and “Physical Layer Procedures,” 3GPP TS.213, V11.0.0, 2012-09, which are both incorporated by reference for definition purposes.
However, using dynamic techniques to re-configure a TDD configuration would cause legacy UEs without a dynamic re-configuration capability and new UEs possessing such capability to have different understandings of the TDD DL-UL configuration, since legacy UEs must follow the system information change procedure while new UE would be able to re-configure the TDD DL-UL configuration via dynamic signaling mechanisms such as physical layer signaling, medium access control (MAC) signaling, or radio resource control (RRC) signaling. This could potentially lead to a variety of problems including problems caused by UE measurements as well as Hybrid Automatic Repeat Request (HARQ) operations.
HARQ is referred to as a transmission technique widely used in modern wireless communication systems. HARQ operates by re-transmitting an identical copy of the original transmission or another redundancy version upon transmission error. The receiver then combines the previously corrupted transmissions with the retransmitted one. In LTE TDD systems, the timing relation between the feedback information indicating a transmission error and corresponding retransmission are separately and differently defined for each of the 7 configurations due to the different allocation of the DL-UL subframes. However, sudden changes of TDD configuration could cause interferences of the HARQ operation between legacy UEs and UEs having the dynamic re-configuration capability.
As legacy UEs (before release 12) are not compatible with the technique of dynamic TDD DL-UL reconfiguration, a new design could be required in order to avoid the possible conflicts between legacy UEs and new UEs (release 12 and beyond).